Black walnut: one of the bad guys in oak savanna restoration

One of the important parts of our savanna restoration work has been the removal of a large number of black walnuts (Juglans nigra). Although black walnut is considered a cash crop in forestry, it is an invasive species as far as we are concerned.

Walnut was absent from the original land surveyor’s records for our area, indicating that it was only a minor component of the original vegetation. Because it is relatively fire sensitive, it would not have been a major component of the savanna. Black walnut is primarily a tree of the mid-continent of the United States and is at the northern edge of its range in southern Wisconsin. Here it is usually found in mixed forests as single isolated specimens, although in some restricted areas it may become dominant or subdominant.

Black walnut toxicity Black walnut produces a toxic substance called juglone (5 hydroxy-1,4-napthoquinone), which occurs in all parts of the black walnut plant. Juglone inhibits the growth of many other plants, a phenomenon called allelopathy. However, tests have shown that many plants are resistant to juglone and are thus able to grow in the presence of black walnuts. Resistant plants include all grasses, and a number of woodland forbs, including jack-in-the-pulpit, bellflower, bellwort, dutchman’s breeches, wild geranium, mayapple, solomon’s seal, bloodroot, and trillium. Also, spiderwort, a common prairie plant, is resistant to juglone. Honeysuckle, an invasive shrub, is also resistant.

Walnut removal at Pleasant Valley We had three areas at Pleasant Valley Conservancy that had large populations of black walnut. The first two were Units 18 and 21, that are on opposite-facing hills that had the same geology. These hills had been in pasture until the mid 1950s, after which they remained as open land. Both hills had some very large bur oaks that had been unaffected by grazing. After grazing ceased, these two hills gradually filled in with woody vegetation, of which black walnut was the most common tree.

The walnuts on Unit 18 were removed in the winter of 2000-2001, leaving behind a handsome stand of mature bur oaks that had survived years of grazing pressure.

The residual selective toxicity of juglone after walnuts are removed may influence the kinds of plants able to become established. It is interesting that in Unit 18, spiderwort, a species known to be tolerant to juglone, developed extensively on the lower slope after the walnuts had been removed, and has remained in large numbers ever since.

Heavy seeding of Unit 18 with native species was done in the fall of 2001, and again in 2003. Also, Unit 18 has been burned annually ever since, and these burns have been very successful burn. Although brambles were a problem, they were dealt with (cut & treat) in 2006 and this hillside has become a fine oak savanna.

Unit 21, on the hill opposite Unit 18, is in the same geological formation and has had the same land-use history. When restoration began in January 2004, the principal trees were black walnut and slippery elm, with a number of small- to medium-sized oaks. The understory was almost pure honeysuckle.

For the Unit 21 work, cold weather and moderate snow cover was favorable for walnut removal, and all of the trees were off the site within a two-week period. Many of the walnut specimens were fairly large, and were good candidates for saw logs (lumber). We took advantage of the solidly frozen ground and had these logs brought down to the town road by a log skidder. Care was taken to avoid disturbance of the soil. There was also a smaller area of large walnut trees in the ravine between Units 12A and 20 and these were cut and skidded over the snow to the road at the same time.

An area of walnuts in Unit 21. There were 25 walnuts here large enough to convert into lumber.

In total, we had 25 logs ranging in length from 9-10 ft and diameter from 14-25 inches. They were taken by a logging truck to a nearby sawmill. (These logs were donated to the Aldo Leopold Foundation, which converted them to lumber for use in their Legacy Center.)

The third walnut area was the marsh that became the Sandhill Crane Wet Prairie. There were over 70 large walnuts in this area, which were cut and donated to a Mount Horeb resident who burned wood as the main source of heat in his home. During a very cold spell in January 2004, this resident used a Bobcat to bring all these logs up to the town road. (This was a barter arrangement: the resident got the logs gratis but did not charge for the extensive work involved.)

Truck load of walnut logs removed from the marsh edge. Over 70 trees were cut at this location.

The Resprout Problem. For most tree species, treatment of the cut stumps with 21% glyphosate prevents resprouting. However, this procedure was not effective for walnuts and extensive resprouting of cut stumps occurred. By the summer of 2002 the walnut stumps on Unit 18 (cut in the winter of 2001) had resprouted extensively. From each stump as many as 20 new shoots had developed, most from the root collar but some from the side of the stump. In January 2003 all of these resprouts were again cut and treated with herbicide. However, many of these stumps resprouted again in the summer of 2003.

It turned out that glyphosate is not too effective with walnut. Switching to triclopyr (Garlon 4; 20% in oil) was the solution. In addition to treating the cut stems, basal bark application was done. Continual monitoring for walnut resprouts has been done, and the problem has been eliminated.